1. Field of the Invention
This invention relates to a compression/decompression method for image data, and more particularly, to a compression/decompression method for image data for controlling the amount of codes to be generated.
2. Description of the Related Art
In conventional compression/decompression methods for image data, control of the amount of codes is performed in units of one frame (screen) using a quantization table in accordance with the JPEG (Joint Photographic Coding Experts Group) standards.
Also systems which control the amount of codes in units of a block of m*n (m and n are positive integral numbers) dots are already known. With the systems, however, since the amount of codes is controlled in units of a block, a large number of code amount control amounts are generated for one frame. It is a possible countermeasure to prepare a header for each frame at a top portion of compressed data after coding and store a large number of code amount control amounts into the header in order to store the large number of code amount control amounts. However, where code amount control amounts are stored in the header, it is difficult to determine to which block data a code amount control amount corresponds. In addition the capacity required for storing code amount control amounts is excessive.
Further, image coding systems are known, wherein image data is divided for individual blocks and an optical image conversion method is selected for each block to effect compression (refer to Japanese Patent Laid-Open Application No. Showa 61-285869). However, since the criterion for selection of which system is optimum is unknown, they are not very practical in use, and increase processing.
In the conventional compression/decompression methods for image data described above, when control of the amount of codes is performed in units of a block, since the number of blocks is varied by the image size, the number of code amount control amounts varies correspondingly. Consequently, the size of a header for storing code amount control amounts varies. The conventional compression/decompression methods are thus disadvantageous in that the management of the headers is complicated and it is inconveniently complicated to establish the corresponding relationship of the individual code amount control amounts to the individual blocks.
Further, where a header is used for storage of code amount control amounts, since code amount control amounts for one frame must be stored, there is a drawback that the header capacity which must be prepared is excessive, resulting in inefficient use of the header capacity.